6v to 12v conversion

Is there something wrong with the alternator that's on the bike now?

You should not need to change the alternator.
See http://www.aoservices.co.uk/info/ALT_6V_12V.pdf
You will need a regulator. A good example is http://www.aoservices.co.uk/products/a-reg.html

And the www.aoservices.co.uk site has a lot of useful information, plus Al with help if you call him.

Just to confirm all the above is correct. An old alternator (prior to mid 60s and 70mm rotor-must be in good condition) 3 wire and set for 6V will give 12V. With the caution, that due to age you might run out of quantity (power/amps) of electricity. But in the first place convert the bike and alternator to 12V and fit electronic Ignition. See how it goes.

Hello Andy, if you look at the Roadholder index on the website you will find numerous articles about 6 - 12v conversion which may help you.
Some are numbers:

89
125
140
200
205
316
342

HtH

Mark Woodward
Roadholder Editor.
 

You connect the GRN/BLK to the GRN/YLW. The alt then becomes a two wire. These two wires feed the rectifier (that you already have) and you fit a Zener diode across the power line.
But Zeners are no longer available, so you replace the rectifier with a regulator/rectifier for 12V (I sell them)  I thought 'every one' knew this. It is on AOSERVICES.CO.UK
The ignition you fit depends on the size of your pocket and what housing you have, the original 18D2 or another magneto replacement housing.